In the past, as a three-level power conversion apparatus, there has been known a three-level inverter with three phases which serves to convert a direct current into an alternating current (for example, see a first patent document).
In the following, a conventional apparatus described in the first patent document will be explained with reference to FIG. 9 through FIG. 20. FIG. 9 is a circuit diagram which shows a general three-level power conversion apparatus described in the first patent document, wherein a main circuit construction of a three-level inverter with three phases is illustrated.
In FIG. 9, the three-level inverter is provided with direct current power supplies 1, 2 which are connected in series with each other, and which have a positive side electric potential P (hereinafter referred to as a “P electric potential”), a negative side electric potential N (hereinafter referred to as an “N electric potential”), and a midpoint electric potential M (hereinafter referred to as an “M electric potential”). Here, note that in cases where the direct current power supplies 1, 2 are constructed by an AC power system, in general, a diode rectifier, an electrolytic capacitor of large capacitance (not shown), etc. are used.
An IGBT (insulated gate type bipolar transistor) modules 16, 17, 18 (main switches) are connected between the P electric potential and the N electric potential, and alternating current output terminals (intermediate terminals) 11 of the individual IGBT modules 16, 17, 18 are connected to the M electric potential through IGBT modules 19 through 24 (bi-directional switches), respectively.
An IGBT (switch element) 3 and a diode 4 in the IGBT module 16 are connected to the P electric potential, and an IGBT 5 and a diode 6 therein are connected to the N electric potential.
Each pair of the IGBT modules 19 through 24 construct a bi-directional switch, which is connected between the M electric potential and each alternating current output terminal (intermediate terminal) 11 in the IGBT modules 16 through 18.
The IGBT module 19 is composed of a set of an IGBT 7 and a diode 8 which is connected in anti-parallel to the IGBT 7, and the IGBT module 20 is composed of a set of an IGBT 9 and a diode 10 which is connected in anti-parallel to the IGBT 9.
The IGBT modules 19, 20 (bidirectional switch) are of a construction in which one set of the IGBT and the diode are connected in anti-series to another set of the IGBT and the diode, and the IGBT modules 21 through 24, which correspond to the other two set, are constructed in a similar manner. Here, note that those IGBTs which are connected in anti-series with each other have their emitters commonly connected with each other, but they may instead have their collectors commonly connected with each other.
The three-level inverter has its three-phase output terminals connected to a load 15 through reactors 12, 13, 14 for filtering.
This serves to construct an inverter which can generate an output of three levels (P electric potential, N electric potential, and M electric potential).
In cases where the three-level inverter of FIG. 9 is constructed by IGBT modules and direct current power supply devices (electrolytic capacitors of large capacitance), for example, the IGBT modules 16, 17, 18 are composed of “2 in 1 type” IGBT modules, respectively, and the IGBT modules 19 through 24 are composed of “1 in 1 type” IGBT modules, respectively, and the direct current power supplies 1, 2 are composed of serially connected electrolytic capacitors, respectively. In addition, the IGBT modules 19 through 24 may be composed of “2 in 1 type” IGBT modules.
FIG. 10 is an external appearance perspective view which shows a construction example of each of the “2 in 1 type” IGBT modules 16 through 18, and FIG. 11 is an equivalent circuit diagram which shows an internal function in FIG. 10.
In FIG. 10 and FIG. 11, the IGBT module is provided with a collector terminal (C1) 27 that is connected to a P electric potential, an emitter terminal (E2) 28 that is connected to an N electric potential, and an intermediate terminal (emitter/collector terminal E1C2) 11 that is connected to a load output and a bi-directional switch. In general, the individual terminals 27, 28, 11 are constructed in the order as shown in FIG. 10.
FIG. 12 is an external appearance perspective view which shows a construction example of each of the “1 in 1 type” IGBT modules 19 through 24 (bi-directional switches), and FIG. 13 is an equivalent circuit diagram which shows an internal function in FIG. 12.
In FIG. 12 and FIG. 13, the IGBT module (bi-directional switch) is provided with a collector terminal (C) 30 and an emitter terminal (E) 31.
On the other hand, FIG. 14 is an external appearance perspective view which shows a construction example of a “2 in 1 type” bi-directional switch, and
FIG. 15 is an equivalent circuit diagram which shows an internal function in FIG. 14.
In FIG. 14 and FIG. 15, IGBT modules (bi-directional switch) are provided with a collector terminal (C1) 40 and a collector terminal (C2) 41.
In cases where the IGBT modules 19 through 24 (bi-directional switches) are composed of modules of the “2 in 1 type”, as shown in FIG. 15, they have a common emitter (or a common collector), and hence, individual terminals can be constructed as shown in FIG. 14.
FIG. 16 is a circuit diagram which shows one phase of the conventional three-level inverter which is described in the first patent document.
In FIG. 16, the IGBT module 16 and electrolytic capacitors 25, 26 are connected to each other, at the side of the P potential, by means of a first conductor 33 which connects between a collector terminal 27 at the side of an upper arm of the IGBT module 16 and a positive side potential terminal 32 of the electrolytic capacitors 25, 26.
In addition, at the side of the N electric potential, the IGBT module 16 and the electrolytic capacitors 25, 26 are connected to each other by means of a second conductor 37 which connects between an emitter terminal 28 at the side of a lower arm of the IGBT module 16 and a negative side electric potential terminal 36 of the electrolytic capacitors 25, 26. At the side of the M electric potential, the IGBT module 16 is connected to a series connection point 34 of the electrolytic capacitors 25, 26 through the IGBT modules 19, 20 (bi-directional switches).
Further, the IGBT modules 19, 20 (bi-directional switches) and the electrolytic capacitors 25, 26 are connected to each other by means of a third conductor 35 that connects between a collector terminal 30 of the IGBT module 19 and the series connection point 34 of the electrolytic capacitors 25, 26.
FIG. 17 through FIG. 19 show the construction of the three-level inverter (for one phase) of FIG. 16. FIG. 17 is a top plan view which shows a state seen from an upper surface thereof. FIG. 18 is a left side view which shows a state seen from a left side thereof. FIG. 19 is s right side view which shows a state seen from a right side thereof.
In FIG. 17 through FIG. 19, in order to distinguish each one pair of series electrolytic capacitors located at the opposite right and left sides, “a” and “b” are attached in such a manner that those electrolytic capacitors which are located at the right side are denoted by 25a, 26a, and those electrolytic capacitors which are located at the left side are denoted by 25b, 26b. 
In FIG. 17 through FIG. 19, the first and the second conductors 33, 37 are arranged in proximity with each other through an insulating material 44.
In addition, an electrically connected two-division conductor 45 (corresponding to the third conductor 35 in FIG. 16) is arranged in the vicinity of an anode (series connection point 34) of the electrolytic capacitor 26, and in the vicinity of the collector terminal 30 of the IGBT module 19, so that a proximity structure is thereby achieved in which the first and the second conductors 33, 37 are sandwiched.
However, insulating materials 46, 48 are interposed between the two-division conductor 45 and the first conductor 33, and between the two-division conductor 45 and the second conductor 37, respectively.
The individual interconductor distances between the first conductor 33 and the two-division conductor 45, between the second conductor 37 and the two-division conductor 45, and between the first conductor 33 and the second conductor 37, respectively, all become δ (see FIG. 18), so that a mutual inductance LM of a large value is generated between the individual conductors.
FIG. 20 is an explanatory view which shows mutual inductances LM by means of an equivalent circuit, wherein in the construction of FIG. 16 through FIG. 19, it is shown that the mutual inductances LM of the same magnitude are generated between individual conductors.
In FIG. 20, the two-division conductor 45 as referred to above (FIG. 18 and FIG. 19) is shown as divided into two, i.e., conductors 45a, 45b. 
The intermediate terminal 11 of the IGBT module 16 and a collector terminal 30a of the IGBT module 20 are connected to each other by means of a thin fourth conductor 42 (wiring inductance Lac).
In addition, an emitter terminal 31a of the IGBT module 20 and an emitter terminal 31b of the IGBT module 19 are connected to each other by means of a thin fifth conductor 43 (wiring inductance Ls).
By forming wiring between the IGBT module 16 and the electrolytic capacitors 25, 26 into a four-layer laminate structure as in the case of the above-mentioned conventional three-level power conversion apparatus (FIG. 16 through FIG. 20) as referred to above (the first patent document), it is possible to reduce the wiring inductances LM between the IGBT module 19 (bi-directional switch) and the electrolytic capacitors 25, 26. However, other wiring inductances Ls, Lac can not be reduced.